Answer:

When heated with ethyl alcohol and a small quantity of sulphuric acid, option D (CH3COOH) will give a pleasant smell of ester. Let's break down the process and explain it in detail:

1. Formation of Ester:
- Esters are organic compounds that are formed by the reaction between an alcohol and a carboxylic acid in the presence of an acid catalyst.
- In this case, ethyl alcohol (C2H5OH) is the alcohol and acetic acid (CH3COOH) is the carboxylic acid.
- When heated with a small quantity of sulphuric acid (H2SO4) acting as a catalyst, the reaction takes place as follows:

C2H5OH + CH3COOH → CH3COOC2H5 + H2O

- The reaction results in the formation of ethyl acetate (CH3COOC2H5), which is an ester.
- Ethyl acetate is a colorless liquid with a fruity smell, often described as having a pleasant fragrance.

2. Role of Sulphuric Acid:
- Sulphuric acid (H2SO4) acts as a catalyst in this reaction.
- A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process.
- In this case, sulphuric acid helps to speed up the esterification reaction between ethyl alcohol and acetic acid.
- It does this by providing protons (H+) that can react with the functional groups of the reactants, promoting the formation of the ester.

3. Reason for the Pleasant Smell:
- The pleasant smell of ester is due to the characteristic aroma of ethyl acetate, which is the ester formed in this reaction.
- Ethyl acetate is commonly used as a flavoring and fragrance compound in various products, including perfumes, cosmetics, and food and beverage items.
- It has a fruity smell, reminiscent of pears or bananas, which is often described as pleasant and sweet.

Conclusion:
When heated with ethyl alcohol and a small quantity of sulphuric acid, CH3COOH (acetic acid) will give a pleasant smell of ester. The reaction between ethyl alcohol and acetic acid, catalyzed by sulphuric acid, forms ethyl acetate, which has a fruity and pleasant fragrance.